Objective lens and scanning device using such an objective lens

ABSTRACT

The invention relates to an objective lens and an optical device for scanning different information carriers having different cover layer thicknesses, with different numerical apertures. The objective lens ( 10, 20 ) comprises at least an annular part ( 101, 201 ) having a first numerical aperture and a central part ( 102, 203 ) having a second numerical aperture. The second numerical aperture is higher than the first numerical aperture.

FIELD OF THE INVENTION

The present invention relates to an objective lens for scanninginformation carriers having transparent layers with differentthicknesses.

The present invention also relates to a scanning device for scanninginformation carriers having transparent layers with differentthicknesses.

The present invention is particularly relevant for an optical discapparatus for reading and/or recording data from and/or to differentoptical discs, e.g. a CD/DVD/BD player and/or recorder.

BACKGROUND OF THE INVENTION

In the field of optical recording, increasing the capacity of theinformation carrier is the trend. The capacity of an information layerdepends, inter alia, on the size of the spot formed by a radiation beamfocused on the information layer. The higher the numerical aperture ofthe radiation beam, the smaller the spot size. As a consequence,different types of information carriers have been developed or are underdevelopment in order to increase the capacity of the informationcarrier. For example, a CD (Compact Disc) has been developed which has acapacity of 700 megabytes and which is scanned by a radiation beamhaving a numerical aperture (NA) of 0.45. A DVD (Digital Video Disc) hasthen been developed which has a capacity of 4.7 gigabytes and which isscanned by a radiation beam having a NA of 0.65. Right now, a BD(Blu-Ray Disc) is being developed which has a capacity of about 25gigabytes and which is scanned by a radiation beam having a NA of 0.85.

Furthermore, when the NA is increased, it is necessary to reduce thethickness of the transparent layer protecting the information layer, soas to reduce the influence of disc tilt on the quality of the radiationbeam. For example, the thickness of the transparent layer of a CD is 1.2millimeters, the thickness of the transparent layer of a CD is 0.6millimeters and the thickness of the transparent layer of a BD is 0.1millimeters.

A compatible player and/or recorder should be able to scan the differenttypes of information carrier. U.S. Pat. No. 6,052,237 describes ascanning device capable of scanning two different types of informationcarriers having different thicknesses of their transparent layer. Thisscanning device comprises a radiation source and an objective lenshaving an outer annular part and a central part inside the annular part.The annular part has a numerical aperture higher than the numericalaperture of the central part.

This scanning device is capable of scanning a first information carrierhaving a first transparent layer with a first thickness and a secondinformation carrier having a second transparent layer with a secondthickness greater than the first thickness. The annular part introducesa first spherical aberration into the radiation beam compensating forthe passage of the radiation beam through the first transparent layer,and the central part introduces a second spherical aberration into theradiation beam compensating for the passage of the radiation beamthrough the second transparent layer.

When the second information carrier is scanned, the radiation beampasses through the central part and the second transparent layer and isfocused on a second information layer. When the first informationcarrier is scanned, the radiation beam passes through the combined areaof the annular part and the central part and through the firsttransparent layer and is focused on a first information layer.

In this scanning device, the spherical aberration introduced by thefirst transparent layer is compensated, because the correction of thecentral part of the objective lens for a thickness of the transparentlayer different from the thickness of the transparent layer for whichthe annular part is corrected has only a relatively small influence.

This may be true when the NA of the annular part is 0.6 and the NA ofthe central part is 0.33, as stated in this patent. However, it is nottrue anymore when the NA of the annular part is increased, for example,to 0.85. As a consequence, such a scanning device cannot be used forscanning, for example, a Blu-Ray Disc and a DVD. Actually, in order touse such an objective lens for scanning a BD and a DVD, the annular partshould have a NA of 0.85. In order to reduce the influence of thecentral part on the radiation beam having a NA of 0.85, the central partshould be as small as possible. Unfortunately, a small central partcannot be used, because the free working distance of the central partwould be too small to cope with the thickness of the transparent layerof a DVD.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an objective lens and ascanning device for scanning different types of information carrierswith increased numerical apertures.

To this end, the invention proposes an objective lens comprising atleast an annular part having a first numerical aperture and a centralpart having a second numerical aperture, wherein the second numericalaperture is higher than the first numerical aperture.

At least a first information carrier having a first transparent layerwith a first thickness and a second information carrier having a secondtransparent layer with a second thickness greater than the firstthickness can be scanned by a scanning device comprising an objectivelens in accordance with the invention. When scanning the firstinformation carrier, a first radiation beam is focused on a firstinformation layer by means of the central part of the objective lens.The first NA may be taken as high as desired, as a function of theinformation carriers intended to be scanned by the scanning device. Forexample, the first NA may be 0.85, in order to scan a BD Disc. Whenscanning the second information carrier, a second radiation beam isfocused on a second information layer by means of the combined areas ofthe annular part and the central part of the objective lens. Only thepart of the radiation beam passing through the annular part of theobjective lens is focused on the second information layer. However, whenscanning the second information layer with the second numericalaperture, the central part of the second radiation beam may be dispensedwith without affecting the quality of the scanning, because the secondnumerical aperture is relatively low compared with the first numericalaperture. For example, the second numerical aperture is 0.65.

If, as in the prior art, the first numerical aperture is lower than thesecond numerical aperture, the quality of the scanning would be affectedwhen scanning an information layer with a relatively high NA through thecombined area of the annular part and the central part of the objectivelens. Actually, a radiation beam having a high NA, such as 0.85, is verysensitive, and the absence of the central part of such a beam leads to arelatively bad scanning.

Advantageously, the objective lens comprises an optical axis and acavity located around said optical axis, said cavity having asubstantially cylindrical shape, the bottom of said cavity forming thecentral part of the objective lens.

Such an objective lens is particularly advantageous, as it can berelatively small. Actually, if a radius of the annular part isrelatively small, the radius of the central part is even smaller. As aconsequence, the free working distance of the central part is small andcan be smaller than the width of the objective lens in the direction ofits optical axis. However, in such an objective lens having a cavity,the central part of the objective lens can be placed near the outputsurface of the objective lens. As a consequence, an information carrierwith a relatively small thickness of the transparent layer can bescanned, even if the free working distance of the central part is small.

Furthermore, such an objective lens is relatively easy to manufacture,as it can be moulded. A mould might be manufactured that has a profilesuitable for producing the objective lens when an optical material, suchas glass or plastic, is introduced in the mould. Hence, a large quantityof objective lenses can be manufactured with a high accuracy by means ofthe same mould.

The invention also relates to a lens assembly comprising a first lenswith an annular part having a first numerical aperture and a centralpart, and a second lens, the second lens and the central part of thefirst lens forming a dual-element objective lens having a secondnumerical aperture, wherein the second numerical aperture is higher thanthe first numerical aperture.

Such a lens assembly advantageously replaces the objective lens asdescribed hereinbefore. Instead of the central part of the objectivelens, a dual-element objective lens is used. This is particularlyadvantageous when the lens assembly is made from plastic, which has alow refractive index. Actually, in the objective lens as describedhereinbefore, the curvature of the central part is high when the NA ofthe central part is high. This requires a relatively high accuracyduring the manufacturing process, which is not required when adual-element objective lens is used.

The invention also relates to an optical scanning device for scanning atleast a first type of information carrier having a first informationlayer and a first transparent layer of a first thickness and a secondtype of information carrier having a second information layer and asecond transparent layer of a second thickness greater than the firstthickness, said optical scanning device comprising means for generatingat least a first and a second radiation beam and an objective lenscomprising at least an annular part having a first numerical apertureand a central part having a second numerical aperture higher than thefirst numerical aperture, wherein the first information layer isintended to be scanned by the first radiation beam through the centralpart of the objective lens and the first transparent layer and thesecond information layer is intended to be scanned by the secondradiation beam through the annular part of the objective lens and thesecond transparent layer.

The invention also relates to an optical scanning device for scanning atleast a first type of information carrier having a first informationlayer and a first transparent layer of a first thickness and a secondtype of information carrier having a second information layer and asecond transparent layer of a second thickness greater than the firstthickness, said optical scanning device comprising means for generatingat least a first and a second radiation beam and a lens assemblycomprising a first lens with an annular part having a first numericalaperture and a central part, and a second lens, the second lens and thecentral part of the first lens forming a dual-element objective lenshaving a second numerical aperture higher than the first numericalaperture, wherein the first information layer is intended to be scannedby the first radiation beam through the dual-element objective lens andthe first transparent layer, and the second information layer isintended to be scanned by the second radiation beam through the annularpart of the first lens and the second transparent layer.

These and other aspects of the invention will be apparent from and willbe elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example,with reference to the accompanying drawings, in which:

FIGS. 1 a and 1 b show an objective lens in accordance with theinvention;

FIG. 2 shows another objective lens in accordance with the invention;

FIGS. 3 a and 3 b show a lens assembly in accordance with the invention;and

FIG. 4 shows a scanning device in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

An objective lens in accordance with the invention is depicted in FIGS.1 a and 1 b. Such an objective lens 10 comprises an annular part 101 anda central part 102. In FIG. 1 b, the objective lens 10 is used forscanning a first information carrier 11 comprising an information layer111 and a transparent layer 112. In FIG. 1 a, the objective lens 10 isused for scanning a second information carrier 12 comprising aninformation layer 121 and a transparent layer 122.

In the example to be described below, the first information carrier 11is a BD scanned by a first radiation beam 13 and the second informationcarrier 12 is a DVD scanned by a second radiation beam 14. The firstradiation beam 13 has a first wavelength of 405 nanometres. The secondradiation beam 14 has a second wavelength of 650 nanometres. Thethickness of the first transparent layer 112 is 0.1 millimetres and thethickness of the second transparent layer 122 is 0.6 millimetres. Theradius of the annular part 101 is 1.8 millimetres and the radius of thecentral part 102 is 0.5 millimetres. The NA of the annular part 101 is0.65 and the NA of the central part 102 is 0.85. The focal length of theannular part 101 is 2.75 millimetres and the focal length of the centralpart 102 is 0.58 millimetres.

When the first information layer 111 is scanned, the first radiationbeam 13, which is a parallel beam having a diameter substantially equalto the diameter of the central part 102, passes through the central part102 and is focused on the first information layer 11 1, through thefirst transparent layer 112. The objective lens 10 can be moved alongits optical axis in order to obtain an accurate focus. As the centralpart has a relatively small radius and a relatively high NA, the freeworking distance of the central part 102, which represents the maximumpossible distance between the output surface of the central part 102 andthe surface of the transparent layer 112, is relatively small. In thisexample, the free working distance of the central part 102 is about 0.4millimetres. However, according to the invention, this is not a problem,because the central part 102 of the objective lens 10 is used forscanning information carriers having transparent layer of a smallthickness. As a consequence, the NA of the central part 102 can be high,for example higher than 0.7 or even higher than 0.8.

When the second information layer 121 is scanned, the second radiationbeam 14, which is a parallel beam having a diameter substantially equalto the diameter of the annular part 101, passes through the combinedareas of the annular part 101 and the central part 102. Compared withthe scanning of the first information layer 111, the objective lens 10is moved along its optical axis in a direction opposed to the firstinformation layer 111 when a second information layer is scanned. Thisis possible, because the free working distance of the annular part 101of the objective lens 10 is relatively great, as the radius of theannular part 101 is great and the NA of the annular part 101 is low. TheNA of the annular part 101 is preferably between 0.35 and 0.7, but maybe higher if the NA of the central part 102 is even higher. For example,the NA of the central part 102 may be higher than 0.9, 1, 1.1 or 1.2. Inthese cases, the NA of the annular part 101 may be 0.7, 0.8, 0.9 or 1,respectively, for example. Preferably, the NA of the annular part 101 ismore than ten percent lower than the NA of the central part 102.

When the second information layer 121 is scanned, the outer portion ofthe second radiation beam 14, corresponding to the portion of the secondradiation beam 14 passing through the annular part 101, is focused onthe second information layer 121. The central portion of the secondradiation beam 14, corresponding to the portion of the second radiationbeam 14 passing through the central part 102, is not focused on thesecond information layer 121. As a consequence, this central portion ofthe second radiation beam 14 is not used for scanning the secondinformation layer 121. However, this does not affect the scanning,because the NA of the second radiation beam 14 is relatively small, sothat the signal read or written to or from the second information layer121 is not affected by the absence of a central portion of the secondradiation beam 14.

It is important to note that the objective lens of FIGS. 1 a and 1 b canbe used for scanning more than two different types of informationcarriers. For example, this objective lens may be used for scanning aCD. In order to scan a CD, the annular part 101 of the objective lens 10is divided into a first annular area having a NA equal to 0.45 and asecond annular area having a NA equal to 0.6. The first annular area islocated near the optical axis of the objective lens 10. Such an annularpart 101 may be used for scanning a CD and a DVD, as explained in U.S.Pat. No. 6,052,237, which does not apply to an annular part divided intotwo annular areas, but to a lens divided into two areas. As aconsequence, such an objective lens can be used for scanning a CD, a DVDor a BD. Another objective lens according to the invention, which may beused for scanning a CD, a DVD or a BD, is depicted in FIG. 2.

FIG. 2 shows another objective lens in accordance with the invention.Such an objective lens 20 comprises a first annular part 201, a secondannular part 202, and a central part 203.

The radius r3 of the first annular part 201 is 1.8 millimetres, theradius r2 of the second annular part 202 is 1.2 millimetres, and theradius r1 of the central part 203 is 0.5 millimetre. The NA of the firstannular part 201 is 0.45, the NA of the second annular part 202 is 0.65,and the NA of the central part 203 is 0.85.

When a CD is scanned by means of the objective lens 20, a thirdradiation beam having a wavelength of 785 nanometres and a radiussubstantially equal to r3 passes through the combined areas of the firstannular part 201, the second annular part 202 and the central part 203.Only the portion of the third radiation beam passing through the firstannular area 201 is focused on an information layer of the CD. However,this does not affect the scanning, as explained hereinbefore, becausethe NA of the third radiation beam is low.

When a DVD is scanned by means of the objective lens 20, a secondradiation beam having a wavelength of 650 nanometres and a radiussubstantially equal to r2 passes through the combined areas of thesecond annular part 202 and the central part 203. As explained withreference to FIGS. 1 a and 1 b, the scanning is not affected by the factthat only the portion of the second radiation beam passing through thesecond annular part 202 is used for the scanning.

When a BD is scanned by means of the objective lens 20, a firstradiation beam having a wavelength of 405 nanometres and a radiussubstantially equal to r1 passes through the central part 203 and isfocused on an information layer of the BD.

FIGS. 3 a and 3 b show a lens assembly in accordance with the invention.Such a lens assembly comprises a first lens 30 comprising an annularpart 301 and a central part 302, and a second lens 31. The central part302 of the first lens 30 and the second lens 31 are combined in order toform a dual-element objective lens. This dual-element objective lens hasa numerical aperture of to 0.85. The annular part 301 of the first lens30 has a numerical aperture of 0.65.

When the first information layer 111 is scanned, the first radiationbeam 13 passes through the dual-element objective lens and is focused onthe first information layer 111, through the first transparent layer112. When the second information layer 121 is scanned, the secondradiation beam 14 passes through the combined areas of the annular part301 and the dual-element objective lens.

As explained in the description of FIGS. 1 a and 1 b, the lens assemblyof FIGS. 3 a and 3 b may be used for scanning more than two differenttypes of information carrier.

Compared with the objective lens of FIGS. 1 a and 1 b, the lens assemblyis easier to manufacture. Actually, manufacturing a single-elementobjective lens having a high NA requires a high accuracy during themanufacturing process, which is not the case with a dual-elementobjective lens, because the curvatures of the two elements can be lowerthan the curvature of a single element.

FIG. 4 shows a scanning device in accordance with the invention. Such anoptical scanning device comprises a first radiation source 401 forproducing a first radiation beam 403, a second radiation source 402 forproducing a second radiation beam 404, a first beam splitter 405, acollimator lens 406, a second beam splitter 407, an objective lens 408,a servo lens 409, and detecting means 410. This optical device isintended for scanning an information carrier 411 comprising aninformation layer 412 and a transparent layer 413.

In the example depicted in FIG. 4, the information carrier 411 is a DVD.The information layer 412 is scanned by the second radiation beam 404produced by the second radiation source 402. The second radiation beam404 has a second wavelength equal to 650 nanometres. The collimator lens406 and the objective lens 408 focus the second radiation beam 404 onthe information layer 412 through the transparent layer 413 having athickness of 0.6 millimetre. The objective lens 408 is the objectivelens 10 of FIGS. 1 a and 1 b. Instead of the objective lens 10 of FIGS.1 a and 1 b, the lens assembly of FIG. 3 a and 3 b may be used as theobjective lens 408.

When a different information layer is to be scanned, such as a BD disc,this information layer is scanned by the first radiation beam 403produced by the first radiation source 401. The first radiation beam 403has a first wavelength equal to 405 nanometres. In order to achieve thescanning of a BD disc, the objective lens is moved in the direction ofthe information carrier 411 by means of an actuator not shown in FIG. 4.The scanning device is designed such that the diameter of the secondradiation beam 404 is substantially equal to the diameter of the annularpart of the objective lens 408, and the diameter of the first radiationbeam 403 is substantially equal to the diameter of the central part ofthe objective lens 408.

The second radiation beam 404, reflected by the information layer 412,is transformed into a parallel beam by the objective lens 408, and thenreaches the servo lens 409, via the second beam splitter 407. Thisreflected beam then reaches the detecting means 410, which, for example,are capable of detecting a focus error signal. This also applies to thefirst radiation beam 403, when a BD disc is scanned.

It is important to note that the scanning device of FIG. 4 may be usedfor scanning more than two different types of information carriers. Forexample, a third radiation source may be provided in the opticalscanning device, said third source being able to produce a thirdradiation beam having a third wavelength equal to 785 nanometres. Thisthird radiation beam may be used for scanning a CD. In this case, theobjective lens 20 of FIG. 2 may be used as the objective lens 408.

Any reference sign in the following claims should not be construed aslimiting the claim. It will be obvious that the use of the verb “tocomprise” and its conjugations does not exclude the presence of anyother elements besides those defined in any claim. The word “a” or “an”preceding an element does not exclude the presence of a plurality ofsuch elements.

1. An optical scanning device for scanning at least a first type ofinformation carrier (11) having a first information layer (111) and afirst transparent layer (112) of a first thickness and a second type ofinformation carrier (12, 411) having a second information layer (121,412) and a second transparent layer (122, 413) of a second thicknessgreater than the first thickness, said optical scanning devicecomprising means (401, 402) for generating at least a first and a secondradiation beam (403, 404), and an objective lens (10, 20, 408)comprising at least an annular part (101, 201) having a first numericalaperture and a central part (102, 203) having a second numericalaperture higher than the first numerical aperture, wherein the firstinformation layer is intended to be scanned by the first radiation beamthrough the central part of the objective lens and the first transparentlayer, and the second information layer is intended to be scanned by thesecond radiation beam through the annular part of the objective lens andthe second transparent layer.
 2. An optical scanning device as claimedin claim 1, wherein said objective lens comprises an optical axis and acavity located around said optical axis, said cavity having asubstantially cylindrical shape, the bottom of said cavity forming thecentral part of the objective lens.
 3. An optical scanning device asclaimed in claim 1, wherein the second numerical aperture is higher than0.7 and the first numerical aperture is more than ten per cent lowerthan the first numerical aperture.
 4. An optical scanning device forscanning at least a first type of information carrier having a firstinformation layer and a first transparent layer of a first thickness anda second type of information carrier having a second information layerand a second transparent layer of a second thickness greater than thefirst thickness, said optical scanning device comprising means forgenerating at least a first and a second radiation beam, and a lensassembly comprising a first lens (30) with an annular part (301) havinga first numerical aperture and a central part (302), and a second lens(31), the second lens and the central part of the first lens forming adual-element objective lens having a second numerical aperture higherthan the first numerical aperture, wherein the first information layeris intended to be scanned by the first radiation beam through thedual-element objective lens and the first transparent layer, and thesecond information layer is intended to be scanned by the secondradiation beam through the annular part of the first lens and the secondtransparent layer.
 5. An objective lens (10, 20) comprising at least anannular part (101, 201) having a first numerical aperture and a centralpart (102, 203) having a second numerical aperture, wherein the secondnumerical aperture is higher than the first numerical aperture.
 6. Anobjective lens as claimed in claim 5, said lens comprising an opticalaxis and a cavity located around said optical axis, said cavity having asubstantially cylindrical shape, the bottom of said cavity forming thecentral part of the objective lens.
 7. An objective lens as claimed inclaim 5, wherein the second numerical aperture is higher than 0.7 andthe first numerical aperture is more than ten per cent lower than thefirst numerical aperture.
 8. An objective lens as claimed in claim 5,wherein the first numerical aperture is between 0.35 and 0.7 and thesecond numerical aperture is higher than 0.7.
 9. An objective lens asclaimed in claim 5, wherein the first numerical aperture is between 0.35and 0.7 and the second numerical aperture is higher than 0.8.
 10. A lensassembly comprising a first lens (30) with an annular part (301) havinga first numerical aperture and a central part (302), and a second lens(31), the second lens and the central part of the first lens forming adual-element objective lens having a second numerical aperture, whereinthe second numerical aperture is higher than the first numericalaperture.